OFDM Relay Systems Research Articles

SER optimization in transparent OFDM relay systems in the presence of dual nonlinearity

Transparent relay is a frequently used solution to enhance signal coverage with low implementation cost. However, the nonlinear distortion caused by cascaded amplification in the relay systems degenerates the system performance, increases the analysis complexity, and brings challenges to the system-level optimization. This paper analytically studies the system-level optimization of symbol error rate (SER) performance in nonlinear OFDM relay systems, where both power amplifiers (PAs) in transmitter and transponder show clear nonlinearity. Firstly, the system SER is expressed as a function of receiver signal-to-noise ratio (SNR), and the latter is further simplified to a polynomial-based function of PA parameters and system operating points. Secondly, the asymptotic behavior and Hessian Matrix (HM) of the receiver SNR are analyzed, and it is found that the SNR has an upper bound when dual nonlinearity of the transmitter PA and transponder PA are considered. Thirdly, this paper derives the analytical expressions of the optimal PA operating points corresponding to the optimal SER performance for the first time. Finally, the expression of the SNR upper bound is presented, and the derived functions are found to have the merit of low complexity, which suits the application in transparent relay systems with limited computational resources.

Asynchronous Physical-layer Network Coding Scheme for Broadband Two-Way Relay Channels

In OFDM based two-way relay channels, carrier frequency offsets (CFOs) between relay and terminal nodes introduce severe inter-carrier interference which degrades the performance of traditional physical-layer network coding. In this paper, we present a two-step asynchronous physical-layer network coding scheme, which consists of an interference cancelation step and a network coding mapping step, to deal with the frequency asynchrony in two-way OFDM relay system. We show through the average signal-to-interference-plus-noise ratio (SINR) and bit error rate (BER) that the proposed scheme can efficiently map the received superimposed signal, which is corrupted by the CFOs, into the exclusive OR of two terminals' transmitted symbols. Moreover, a simple upper bound of the average SINR for the proposed scheme is derived. Through the simulation results, it is shown that the exact average SINR curve of the proposed scheme is very close to the upper bound even when the normalized CFO is as large as 0.15. Finally, we show that the average SINR and BER performances of the proposed scheme outperform the reference schemes considerably.

Performance Analysis of Dual-Hop OFDM Relay System with Subcarrier Mapping in Nakagami-<i>m</i> Fading

Performance Analysis of Dual-Hop OFDM Relay System with Subcarrier Mapping in Nakagami-<i>m</i> Fading

Adaptive Band Activity Ratio Control with Cascaded Energy Allocation for Amplify-and-Forward OFDM Relay Systems

Adaptive Band Activity Ratio Control with Cascaded Energy Allocation for Amplify-and-Forward OFDM Relay Systems

QoS‐driven jointly optimal subcarrier pairing and power allocation for OFDM amplify‐and‐forward relay systems

SUMMARYIn this paper, we investigate the quality‐of‐service (QoS) driven subcarrier pairing and power allocation for two‐hop amplify‐and‐forward OFDM relay systems. By integrating the concept of effective capacity, our goal is to maximize the system throughput subject to a given delay QoS constraint. We propose a jointly optimal subcarrier pairing and power allocation scheme, which can be implemented with two separate steps. First, pair the subcarriers over the source‐relay channel and relay‐destination channel by the descending order of the subcarriers’ channel gains. Second, by making use of the derived equivalent end‐to‐end channel gains of the subcarrier pairs, optimally allocate power over the subcarrier pairs, and then optimally partition the power of the subcarrier pairs between the source and the relay. The simulation results show that our proposed scheme can efficiently provide different levels of delay QoS guarantees, even if under stringent delay QoS constraints. The simulation results also verify that our proposed scheme shows significant superiorities over the other existing schemes. Copyright © 2013 John Wiley & Sons, Ltd.

Adaptive Subcarrier Allocation for OFDM Relay System

This paper proposes two scheduling algorithms for subcarrier allocation of OFDM relay system, enhanced two-hop adaptive proportional fairness (E-THAPF) and user-number-dynamic two-hop adaptive proportional fairness (UND-THAPF), aiming to improve the performance of the traditional two-hop proportional fairness (THPF) scheduling algorithm. By modifying the priority of the two hops in THPF in an adaptive fashion, E-THAPF and UND- THAPF can optimize the subcarrier allocation dynamically despite of the subcarriers fluctuation, and thus can keep the system flexible and stable. Moreover, the factor α in the dynamic index of UND-THAPF is made adaptive to the change of the number of users. Simulation results show that the proposed algorithms achieve a better compromise between system fairness and spectral efficiency. In particular, while E-THAPF is designed to be adaptive to different sub-channel conditions, UND-THAPF performs even better for taking number of users into account in the optimization. 

QoS-Driven Jointly Optimal Subcarrier Pairing and Power Allocation for Decode-and-Forward OFDM Relay Systems

In this paper, we investigate the quality-of-service (QoS) driven subcarrier pairing and power allocation for two-hop decode-and-forward (DF) OFDM relay systems. By integrating the concept of effective capacity, our goal is to maximize the system throughput subject to a given delay-QoS constraint. Based on whether the destination can receive the signal transmitted by the source, we consider two scenarios, i.e. OFDM DF relay systems without diversity and OFDM DF relay systems with diversity, respectively. For OFDM DF relay systems without diversity, we demonstrate that the jointly optimal subcarrier pairing and power allocation can be implemented with two separate steps. For OFDM DF relay systems with diversity, we propose an iterative algorithm to achieve jointly optimal subcarrier pairing and power allocation. Furthermore, we find that the analytical results show different conclusions for the two types of OFDM relay systems. For OFDM relay systems without diversity, the optimal power allocation depend on not only the channel quality of subcarriers but also the delay QoS constraints, while the optimal subcarrier pairing just depends on the channel quality of subcarriers. For OFDM relay systems with diversity, both the optimal subcarrier pairing and power allocation depend on the channel quality of subcarriers and the delay QoS constraints. Simulation results show that our proposed scheme offers a superior performance over the existing schemes.

Resource allocation for OFDM relay systems with statistical QoS guarantees

SUMMARYBy integrating the concept of effective capacity, we propose the resource allocation schemes for subcarrier‐pair based OFDM decode‐and‐forward and amplify‐and‐forward relay systems for quality‐of‐service (QoS) guarantees. The objective is to maximize the system throughput subject to a given statistical delay QoS constraint. First, we pair the subcarriers over the source‐relay channel and the relay‐destination channel by the descending order of the subcarriers' channel gains. Second, by making use of the derived equivalent end‐to‐end channel gains of the subcarrier pairs, we apply joint water‐filling power allocation over the subcarrier pairs and then partition the power of the subcarrier pairs between the source and the relay. We prove that as the equivalent end‐to‐end channel gains of the subcarrier pairs are given, the combination of sorted subcarrier pairing and joint water‐filling power allocation is jointly optimal to maximize the effective capacity. The simulation results show that our proposed schemes achieve the highest effective capacity for OFDM decode‐and‐forward and amplify‐and‐forward relay systems as compared with other existing schemes. The results also verify that our proposed schemes can efficiently provide different levels of delay QoS guarantees, even if under the stringent delay QoS constraints. Copyright © 2012 John Wiley & Sons, Ltd.

Selective Subcarrier Pairing and Power Allocation for DF OFDM Relay Systems with Perfect and Partial CSI

This paper investigates a decode-and-forward two-hop relaying system consisting of one source, one relay and one destination, in which orthogonal frequency division multiplexing is used. The relay forwards the message received from the source on a subset of available subcarriers in the second time slot. Firstly, a subcarrier pairing and selection algorithm is proposed, assuming that perfect channel state information (CSI) is available at all nodes, then, power is allocated to both the source and relay stations under individual power constraints in order to maximize the capacity. Secondly, subcarrier selection and pairing, and power allocation (PA) under partial CSI assumption along with individual power constraints are addressed. The result is a novel distributed algorithm with low complexity maximizing the expected value of capacity at the source and relay nodes. Finally, the simulation results show that selective relaying combined with subcarrier pairing and PA improves the system capacity to a considerable extent in both perfect and partial CSI cases.

Capacity Performance Analysis for Decode-and-Forward OFDM Dual-Hop System

In this paper, we propose an exact analytical technique to evaluate the average capacity of a dual-hop OFDM relay system with decode-and-forward protocol in an independent and identical distribution (i.i.d.) Rayleigh fading channel. Four schemes, (no) matching “and” or “or” (no) power allocation, will be considered. First, the probability density function (pdf) for the end-to-end power channel gain for each scheme is described. Then, based on these pdf functions, we will give the expressions of the average capacity. Monte Carlo simulation results will be shown to confirm the analytical results for both the pdf functions and average capacities.